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For Tropical Rainfall, Timing Is Everything

May 20, 2013

by Richard Merritt

DURHAM, N.C. -- With changes in the global climate, certain regions of the tropics will likely experience growing unpredictability of their seasonal cycles of rain and drought, according to a new analysis by Duke and Princeton University engineers.

While life in these regions has adapted to these cycles, an analysis of more than 80 years of rainfall data has found that the variability of precipitation is becoming more unpredictable over time, which could have dire consequences for agriculture and economies in the tropics.

The researchers studied worldwide rainfall statistics and found three areas where the trends are the most alarming -- northeast Brazil, western and central Africa, and northern Australia. These regions experience extreme seasonal cycles in rainfall -- months with no rain, then periods of high rainfall. They said a better understanding of this variability can help policymakers plan effective use of scarce resources.

"While the debate over climate change tends to focus on temperature, we have demonstrated that changes in rainfall -- especially its seasonal intensity, timing and duration -- can be just as important,” said Xue Feng, a graduate student in the laboratory of senior researcher Amilcare Porporato, professor of civil and environmental engineering in Duke’s Pratt School of Engineering. She is the first author of a paper published online in the journal Nature Climate. Princeton’s professor Ignacio Rodriguez-Iturbe was the other collaborator.

“We project that this increased variability, combined with changing seasonal trends, if sustained into next century, could lead to significant shifts in these ecosystems,” Feng said.

Porporato added, “Climate change has altered not only the overall magnitude of rainfall in these regions, but also its seasonal distribution worldwide. Most rainfall analyses tend to focus on annual amounts of rainfall, whereas we looked at the variability throughout the year, which can have dramatic effects on life in these areas.”

The data collected by the researchers came from 2,175 weather stations in the Global Historical Climatology Network. The three regions studied -- all between 20 degrees north and south of the equator -- were selected for study because their temperatures are relatively similar.

The researchers analyzed monthly rainfall with several statistical techniques, providing a more detailed picture of the timing and duration of rainfall throughout the tropics. Over the timescale they examined, they found statistically significant changes in the trends and seasonal variability for the locations studied.

“In the tropics, rainfall variability has created an array of different ecosystems, from dry forests to open woodland forest to savannas, which support species that have adapted in their own ways,” Feng said. “Most of these ecosystems are extremely sensitive to not only the yearly rainfall amount, but also the timing.”

For example, the arrival of rainfall at the beginning of the rainy season plays a key role in determining when plants start flowering.

“This rainfall seasonality, with its associated drought and flood risks, also poses a huge challenge to local populations, making agricultural efforts and sustainable management of soil and water resources more difficult,” Porporato said.

“The framework we have developed can be broadly adapted not only to seasonally dry tropical regions, but also to the Mediterranean and areas subject to monsoons, where rainfall is unevenly distributed,” Porporato said. “The results can guide strategies for mitigating water shortages and addressing ecosystem vulnerabilities.”

The research was funded by the National Science Foundation, Department of Energy, the USDA’s National Institute of Food and Agriculture, and the National Aeronautics and Space Administration.